It has been known for at least 30 years, since the discovery of Metglasses (iron based glass forming compositions used for transformer core applications) that iron based alloys could be made to be metallic glasses. However, with few exceptions, these iron based glassy alloys have had very poor glass forming ability and the amorphous state could only be produced at very high cooling rates (>106 K/s). Thus, these alloys can only be processed by techniques which give very rapid cooling such as drop impact or melt-spinning techniques.
All metal glasses are metastable and given enough activation energy they will transform into a crystalline state. The kinetics of the transformation of a metallic glass to a crystalline material is governed by both temperature and time. In conventional TTT (Time-Temperature-Transformation) plots, the transformation often exhibits C-curve kinetics. At the peak transformation temperature, the devitrification (transformation from an amorphous glass to a crystalline structure) is extremely rapid, but as the temperature is reduced the devitrification occurs at an increasingly slower rate. When the crystallization temperature of the metallic glass is increased, the TTT curve is effectively shifted up (to higher temperature). Accordingly, any given temperature will be lower on the TTT curve indicating a longer devitrification rate and, therefore, a more stable metal glass structure. These changes manifest as an increase in the available operating temperature and a dramatic lengthening of stable time at any particular temperature before crystallization is initiated. The result of increasing the crystallization temperature is an increase in the utility of the metal glass for a given, elevated service temperature.
Increasing the crystallization temperature of a metal glass may increase the range of suitable applications for metal glass. Higher crystallization temperatures may allow the glass to be used in elevated temperature environments, such as under the hood applications in automobiles, advanced military engines, or industrial power plants. Additionally, higher crystallization temperatures may increase the likelihood that a glass will not crystallize even after extended periods of time in environments where the temperature is below the metal glass's crystallization temperature. This may be especially important for applications such as storage of nuclear waste at low temperature, but for extremely long periods of time, perhaps for thousands of years.
Similarly, increasing the stability of the glass may allow thicker deposits of glass to be produced and may also enable the use of more efficient, effective, and diverse industrial processing methods. For example, when an alloy melt is spray formed, the deposit which is formed undergoes two distinct cooling regimes. The atomized spray cools very quickly, in the range of 104 to 105 K/s, which facilitates the formation of a glassy deposit. Secondarily, the accumulated glass deposit cools from the application temperature (temperature of the spray as it is deposited) down to room temperature. However, the deposition rates may often be anywhere from one to several tons per hour causing the glass deposit to build up very rapidly. The secondary cooling of the deposit down to room temperature is much slower than the cooling of the atomized spray, typically in the range of 50 to 200 K/s. Such a rapid build up of heated material in combination with the relatively slow cooling rate may cause the temperature of the deposit to increase, as the thermal mass increases. If the alloy is cooled below the glass transition temperature before crystallization is initiated, then the subsequent secondary slow cooling will not affect the glass content. However, often the deposit can heat up to 600 to 700° C. and at such temperatures, the glass may begin to crystallize. Thus, this crystallization can be avoided if the stability of the glass (i.e. the crystallization temperature) is increased.
There are many important parameters used to determine or predict the ability of an alloy to form a metallic glass, including the reduced glass or reduced crystallization temperature, the presence of a deep eutectic, a negative heat of mixing, atomic diameter ratios, and relative ratios of alloying elements. However, one parameter that has been very successful in predicting glass forming ability is the reduced glass temperature, which is the ratio of the glass transition temperature to the melting temperature. The use of reduced glass temperature as a tool for predicting glass forming ability has been widely supported by experimentation.
When dealing with alloys in which the glass crystallizes during heating before the glass transition temperature is reached, the reduced crystallization temperature, i.e., the ratio of the crystallization temperature to the melting temperature, can be utilized as an important benchmark. A higher reduced glass transition or reduced glass crystallization temperature indicates a decrease in the critical cooling rate necessary for the formation of metallic glass. As the critical cooling rate is reduced the metallic glass melt can be processed by a larger number of standard industrial processing techniques, thereby greatly enhancing the functionality of the metallic glass.